The present invention relates to a structure of a main combustion chamber formed by a hollow at a top a piston of a direct-injection internal combustion engine.
Conventionally, as shown in FIGS. 1 and 2, a main combustion chamber 2 of a direct-injection internal combustion engine is, generally, of a toroidal shape and is formed by a hollow at a top of a piston 1. As a diameter d of the chamber 2 increases with respect to an inner diameter D of a cylinder, a fuel sprayed from a fuel injection nozzle 10 moves a long distance l. And if the distance increases from a value l1 to l2, a level of stimulative smell of an exhaust degreases in a low speed and low load driving at a same effective compression ratio .epsilon., as shown in FIG. 3. If the compression ratio is increased, the level of the smell is also improved.
However, if the effective compression ratio is increased (if the capacity of the chamber 2 is decreased) in order to improve the stimulative smell of the exhaust in the low speed and low load driving, a maximum power decreases. Further, if the distance for the spraying is increased (if the diameter d is increased) for the same purpose, the speed of the squish decreases. Because, the high compression ratio and the long distance l of the fuel spray cause over-early firing in the high speed and high load driving, and further, the low force of the squish requires a long time for the combustion, so that the maximum power, the color of the exhaust and the fuel consumption deteriorate.
Also, a main combustion chamber 2, as shown in FIG. 2A, has already been used. A hollow 2a forming the chamber 2 is provided with some walls 6 against which the sprayed fuel collides. These collision walls 6 are disposed symmetrically with respect to a center 8 of the piston.
In this chamber, squish flows w into the near of the respective collision walls 6 from circumferentially both sides thereof before the end of the compression process of the engine, so that the fuel spray can be combusted sufficiently. However, a swirl (circumferential flow) of a suction gas can not be utilized for mixing the fuel and the air. Because, the swirl of the gas generated in the chamber 2 collides against and is disappeared by inclined walls 6' and 6' at the both sides of the collision walls 6 which are projected toward the center 8 of the piston. Further, the fuel does not flow along the inclined wall 6' from the near of the wall 6. Therefore, the flow along the wall 6 is not substantially formed in the hollow 2a, and thus, the film of the fuel does not, unpreferably, extend or spread, so that the combustion performance can not be improved.
Japanese patent publication Nos. 51-29242, 51-29243 and 51-29244 have disclosed structures in which the fuel spray is not sprayed against corners of the combustion chamber. In this case, since a radius (r) of curvature of the corner is small (i.e., r/R is in a range from 0 to 0.075: R=radius from the nozzle hole to the collision wall), the fuel spray is accumulated after collision, and thus, the speed of vaporization is low, which causes accumulation of dreg of the fuel and deteriorates the performance.
A Japanese laid-open utility model publication No. 57-168729 and a Japanese patent publication No. 49-16881 have disclosed structures in which collision surfaces are curved or are designed to reflect the fuel spray. However, in these structures, when a small amount of fuel is sprayed, the speed of the fuel jet is very small in comparison with that when a large amount of the fuel is sprayed, so that the fuel is hardly reflected. Therefore, the fuel is accumulated on the wall which is intended to reflect the fuel, which causes, disadvantageously, uncombusted gas and the stimulative smell.
A Japanese utility model laid-open publication No. 57-107821, the Japanese utility model publication No. 55-4515, and the Japanese utility model laid-open publication No. 57-139631 have disclosed structure in which a small amount of fuel can be perfectly combusted in the compressed air in the low power driving. However, according to a shape of a hollow, since a swirl or circumferential flow is braked during the compression, there is such a disadvantage that the sprayed fuel is not actually flowed by the swirl. Further, according to the utility model of the above laid-open publication 57-139631, since the radius of curvature of the wall against which the fuel collides is small, an umcombusted fuel is not spread when the small amount of the fuel is sprayed, so that the gas can not combust sufficiently. Further, since the area of the opening of the combustion chamber is large with respect to the area of the top surface of the piston, which, disadvantageously, reduces the forces of a squish and a swirl of the fuel flow in the hollow.
Accordingly, it is an object of the invention to provide a structure in which the stimulative smell in the low speed and low power driving is improved as well as the maximum power, the color of the exhaust, fuel comsumption and others are also improved in the high speed and high power driving.
According to the invention, a structure of a main combustion chamber of a diesel engine of a direct injection type in which a main combustion chamber having a peripheral wall which includes a plurality of collision walls and guide walls which are disposed alternately to each other in a circumferential direction of the piston, characterized in that; each guide wall includes an upstream portion and a downstream portion with a boundary therebetween in the direction of the swirl, a radial distance from the center of the chamber to the boundary is shorter than a radial distance from the center of the chamber to other portions and each upstream portion has a longer radius of curvature and a circumferential length than each downstream portion so that the inner periphery of the chamber has a pin-wheel-like shape.
Other and further objects, features and advantages of the invention will appear more fully from the following description of the preferred embodiments of the invention.